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Chapter 14 Optical Network Management Alan McGuire Core Transport, Internet & Data Networks, OP7, B29, Adastral Park, Martlesham Haeth, Suffolk, United Kingdom IP5 3RE. Email: alan.mcguire@bt.com Abstract: Optical networks will play an essential role in meeting the demands for future communications bandwidth. With such large traffic volumes at risk network management is fundamental to the running of such networks. To achieve this on an industrial scale management solutions must be based on an unambiguous framework that describes the entities that need to be managed. This chapter describes such a framework and provides a high level summary of how it can be applied to the management of both simple and complex structures. Nevertheless management of this new technology is still at a very early stage and considerable effort is required within the industry before the vision of an optical transport network can be fully realized. 1. INTRODUCTION The accelerating demand for bandwidth fuelled by growth in both the Internet and broadband services represents a major challenge to network operators. Globally operators are facing up to a shortage of fiber capacity in parts of their networks. Whereas this has been most acute in the US where growth in traffic is greatest it is rapidly becoming an international problem. Conventionally an operator would have increased capacity by deploying more fiber or introducing higher bit rate digital systems. Approximately three years ago a new option became commercially available, wavelength division multiplexing. This technology has emerged from the research labs 302 OPTICAL WDM NETWORKS and in a relatively short time frame has become an essential weapon in the transmission engineer’s arsenal. Over 1000 of these systems have now been deployed globally. Depending on the number of channels utilized WDM increases the capacity of a fiber from between 2.5 – 10 Gbps with existing single channel systems to between 40 and 200 Gbps. With large traffic volumes at risk network management is an essential component of such systems. At the present time the management of these systems is relatively simple. In the next few years we can expect to see the introduction of more complex optical systems such as rings and cross-connect meshes. There has been a considerable amount of literature published regarding the transmission characteristics of such networks but very little on the network management challenge. Yet the lack of large scale industrial strength management systems represents a major barrier to the deployment of the optical network. This is particularly true in the existing competitive environment where automation of many of the tasks involved in running and maintaining the network will become a necessity to drive down operating costs and to provide faster provision of services. In this chapter we shall examine some of the features of existing optical network management solutions and describe what will be required in order to manage the optical transport network of the future. But first we need to understand what it is that we want to manage. 2. OPTICAL TRANSPORT NETWORK ARCHITECTURE In order to manage a communications network it is necessary to describe the entities that need to be managed in a rigorous and unambiguous manner. The integration of technology and function is now so great that it is impossible to accurately describe what functions a network element provides by means of a semi-formal technique. Furthermore, the achievement of successful scalable software systems is predicated on a clear definition of what is to be managed and its behaviour. Transport networks can be described in terms of layer networks in accordance with the architectural principles cited in (ITU-T Recommendation G.805, 1995). Each layer network represents a set of inputs and outputs (or access points) that can be interconnected and the layer network is characterized by the information that is transported across it. This information, or characteristic information as it is termed, is a signal of characteristic rate, coding and format. Examples of layer networks include the VC and VP layers in ATM, the VC-12, VC-4, multiplex and the Optical Network Management 303 regenerator sections in SDH. A layer networks topology can be described in terms of subnetworks and links between them, as illustrated in Figure 1. Figure 1: Components of a layer network A subnetwork can be decomposed into smaller subnetworks interconnected by links, in other words it is recursive. This decomposition can, if required go from a global network right down to the smallest subnetwork that is equivalent to a single network elements cross-connect fabric. Connectivity in any layer network can be managed at the network level in the same manner independently of technology. In other words, once we know how to manage connectivity in one layer we should be able to do it for all layers. Managed objects that represent resources within a layer include connection, link, subnetwork, trail, network trail termination points (NW TTPs) and Network Connection Termination Points (NW CTPs). These managed objects represent an abstract view of the resource that can be manipulated by a network manager. Layer networks have client/server relationships with each other, and in many cases one server layer may support different types of clients (Figure 2). An example is the VC-4 network layer which can support (is the server of) VC-3, VC-2-nc, VC-12, ATM VP etc. Whereas it can support these 304 OPTICAL WDM NETWORKS different clients the definition of its characteristic information is separate and distinct. In turn the VC-4 layer network is the client of the multiplex section. To get from one layer to another requires some processing to alter the characteristic information and this is provided by entities known as adaptation and termination functions. Adaptation functions provide functionality such as multiplexing/demultiplexing, frequency justification, timing recovery, alignment and soothing. Termination functions provide means of ensuring signal integrity supervision within a layer by means, for example, of error detection codes, trail trace identifiers, remote indicators and performance monitoring. The transfer of validated information is termed a trail. Figure 2: The client/server relationship TCP – termination connection point, CP – connection point In addition to providing a network view, the architecture can also be applied to network elements where adaptation and termination functions are combined to describe functionality as it can be observed from the inputs and outputs of the network element. The internal structure of the implementation (the equipment design) does not need to be identical to the structure of the functional model as long as the external observable behaviour is the same. Optical Network Management 305 The management view of a network element is based upon an information model containing managed objects that can be manipulated by a management system. The definition of a managed object is derived from a specific part of the functional model. For example, generic trail termination point and connection termination point classes are defined generically such as CTP and TTP, from which technology dependent subclasses such as rsTTP and rsCTP (in SDH) can be developed using the object oriented principle of inheritance. These managed objects have attributes and behaviour that is manipulated from a management system; i.e., they can generate alarms or change their connectivity to other objects. The managed object effectively hides the implementation of the resource that it represents from the management systems and only provides information about aspects of that resource which are important from a network management view. In essence, element managers manipulate managed objects and relationships between these managed objects within a network element, whereas network management is concerned with entities such as network connections, which may use resources from several network elements. The reader is warned however that this is a very simplified picture of the reality. ITU-T Recommendation G.872 (1995), which is a technology dependent version of G.805, defines three optical layers, as shown in Figure 3, in the optical transport network (OTN): – an optical channel (OCh) layer network that provides end-to-end networking of optical channels for transparently conveying digital client information of varying formats (e.g., SDH, PDH and ATM) – an optical multiplex section (OMS) layer network that provides functionality for transport of a multi-wavelength optical signal – an optical transmission section (OTS) layer network that provides functionality for transmission of optical signals on optical media 306 OPTICAL WDM NETWORKS Figure 3: The Optical transport network layers The optical transport network architecture of G.872 is extremely flexible and supports the following features (not all of which may appear in a single instance of a network): – Unidirectional, bidirectional, and point-to-multipoint connections. – Individual optical channels within a multiplex may support different client types. – Cross-connection of optical channels can be accomplished by either wavelength assignment or wavelength interchange. – Optical transport network functionality can be integrated with client functionality in the same equipment. – Interworking between equipment with existing single-channel optical interfaces (ITU-T Recommendation G.957, 1999) and equipment containing optical transport network functionality. Optical Network Management 307 The last two points are significant since they provide network operators with a degree of flexibility in the design of their networks, both now and in the future. Each of these layer networks provides overhead for the operations administration and maintenance of its layer. OAM functions include continuity and connectivity supervision, defect indications (upstream and downstream), protection switching protocols and channels for transporting management information. Not all of these functions are found in each layer. Initially there was considerable debate with regard to the nature of the overhead at the optical channel level as some people viewed the optical channel as a transparent entity within the network, but such an entity, almost by definition cannot be managed. Instead of optical transparency the optical channel provides service transparency, and this is perhaps more central to the concept of optical networking as described by the ITU. The optical channel overhead is created within a digital frame that takes the client layer signal in the form of a continuous data stream and adds both the overhead and a forward error correction mechanism for improving system margin. This frame is known as a digital wrapper. The concept of the digital wrapper can be viewed in one of two ways, firstly as an overhead that is carried end-to-end and so may be switched in optical channel subnetworks. However, this requires that the optical channel remain in the optical domain from beginning to end. Alternatively at intermediate points part, but not all, of the overhead may be processed. At such points the optical channel is regenerated, the FEC is computed and the relevant parts of the overhead are processed. There is however no need to obtain access to the payload. In contrast to all optical networks, optical transport networks utilize the strengths of both electronics and optics to produce much more scalable networks. Nevertheless there is considerable debate within the standards community as to the appropriate overheads and the choice of FEC. The following figures (4, 5 and 6) show the relationships between the resources of the transport network and managed objects. The actual objects are subject to change in the standards’ body and the ones provided here are for indication purposes. With these in hand we have the basic resources of the optical transport network that need to be managed. 308 OPTICAL WDM NETWORKS Figure 4: Relationship between equipment resources and managed objects. The adaptation and termination functions and their connectivity are managed and controlled via managed objects Optical Network Management 309 Figure 5: Relationship between optical transport resources and optical managed objects. The example shown can be considered as an optical channel cross-connect with two ports 310 OPTICAL WDM NETWORKS Figure 6: Entity-Relationship between optical managed objects 3. OPTICAL NETWORK MANAGEMENT ARCHITECTURE Network management is an activity that allows a network operator to administrate, plan, provision, install, maintain, monitor and operate a telecommunications network and its services. Within the ITU-T, the general architecture of the management network is described in terms of a Telecommunications Management Network (TMN) as described in (M.3010, 1996). The TMN concept can be applied to a variety of scenarios including public and private networks, exchanges, digital and analog transmission systems, ISDN, circuit and cell-based networks, operations systems, PBX’s and signalling systems. It can also be applied to optical networking. Although there are some issues concerning the viability of TMN, the principles described below are very general and can be applied to any management architecture. [...]... is not interdependent, and nor should it be It does not prevent them from both being available on a single element manager albeit as separate applications 312 OPTICAL WDM NETWORKS Figure 7: Architecture of management networks An OMN may be subdivided into a set of optical management subnetworks, OMSNs (see Figure 7) In formal terms an OMSN can be defined as a separate set of optical transport network... variety of systems, not just SDH The layer networks of the SDH systems belong to a SDH management subnetwork (SMS) and may be managed by a separate element manager Management communications is available between elements within the optical line system and between the SDH systems but not between the line systems and the SDH systems Optical Network Management 317 Where stand-alone systems are used, it is highly... means of the optical supervisory channel The element manager is considered to be part of the optical management network and can be used to manage optical network elements in a number of OMSNs Large networks will contain many OMSNs and a number of element managers Management workstations are considered to be outside of the OMN but may be located with the OS or remotely An example of a possible management. . .Optical Network Management 311 The basis for the management of the optical network and its network elements is the following simple rule: The management of the optical layers must be separable from its client layers, or, put another way the management of the optical layers is not dependent on a particular client layer even if they... communications is available between all of the network elements, and network faults can be diagnosed in a single step manual process via a single 318 OPTICAL WDM NETWORKS element manager A further advantage is that the data communications network structure is more straightforward than the stand-alone case 7 MANAGEMENT OF OPTICAL RINGS The management of optical line systems is relatively straightforward In addition... between the client circuits, the optical layers, fiber, cable and duct and the location and type of optical network elements In addition, for a network containing a large number of optical line systems the operator may consider introducing an interface between the optical element managers and a network level fault manager for the purpose of fault correlation across technologies and identifying the root cause... cost of optical line systems In this example every network element is also an optical network element and the optical layers are managed within an OMSN whereas the client layer network entities within the end terminals are managed within a SMSN Although these two management subnetworks are logically separate they can be managed from a single element manager In contrast to the stand-alone case, management. .. the optical network can support both existing and future, unforeseen, clients It does not necessarily mean that a link in an optical network simultaneously supports a wide variety of clients, rather it suggests that regardless of which protocol is supported in an instance of an optical network, the architecture and management is the same as in all other instances of an optical network It allows WDM. .. network manager and there can be a hierarchy of such systems Optical Network Management 313 Figure 8 Components of the management system The concepts described above can be understood with reference to a optically protected line system, similar to those being deployed in the BT network, containing line terminals and optical line amplifiers, which is used to interconnect SDH line terminals The optical line... wavelength separate to all of the other optical 314 OPTICAL WDM NETWORKS channels It is inserted and extracted separately from the other optical channels For example in an optical line amplifier the optical supervisory channel is extracted, converted to a digital signal, processed then reinserted, whereas the other channels are amplified in the optical domain A supervisory channel also carries other information . transmission of optical signals on optical media 306 OPTICAL WDM NETWORKS Figure 3: The Optical transport network layers The optical transport network architecture of G.872 is extremely flexible and supports. of TMN, the principles described below are very general and can be applied to any management architecture. Optical Network Management 311 The basis for the management of the optical network and its. albeit as separate applications. 312 OPTICAL WDM NETWORKS Figure 7: Architecture of management networks An OMN may be subdivided into a set of optical management subnetworks, OMSNs (see Figure 7).